Briefly, a rotary hearth furnace (RFH) is a continuous heating furnace generally having a refractory roof supported by an annular inner refractory lined wall that is circumscribed by a spaced annular outer refractory lined wall. The inner wall and the outer wall cooperatively define a circular hearth path along which a rotating hearth travels on wheels. The rotating hearth generally consists of a metal support plate including a top of refractory material and having skirts on either side which project into water-filled troughs to form a non-contact gas seal. The rotary hearth furnace is heated by a plurality of burners spaced around the path of rotation of the hearth.
Material to be treated is usually loaded (dropped) onto the rotating hearth surface by a conveyor or chute projecting downwardly through the roof The material is then conveyed through the rotary hearth furnace by the rotation of the hearth to a discharge area where the treated material is removed from the furnace. For a more detailed discussion of the design of a rotary hearth furnace reference is made to U.S. Pat. No. 4,622,905, incorporated herein by reference.
It will be appreciated that in a typical rotary hearth furnace the refractory is heated to operating temperature by burners located in various sections of the furnace. The feed material is heated in the rotary hearth furnace principally by radiation from the surrounding refractory side walls and roof When the material is loaded in multiple layers, the uppermost exposed layer of feed material heats the fastest and attains the highest temperature. Conversely, the bottom layer that is buried under the uppermost exposed layer attains a lower peak temperature and heats more slowly. Although loading of multiple layers of feed material offers the advantage of greater loading and greater production for the same size furnace, successful treatment of the feed material in a rotary hearth furnace requires that all of the feed material have a similar time versus temperature exposure in the furnace. However, as noted above, too great a variation of loading depth of feed material will typically result in unacceptable feed material time versus temperature uniformity in view of the fact that all of the feed material is typically removed from the hearth when the hearth completes one revolution.
It will be appreciated that uniformity of time versus temperature exposure of the feed material in the rotary hearth furnace is best achieved by treating the feed material in a rotary hearth furnace that is capable of uniformly distributing the feed material that is being loaded onto the rotating hearth. The optimum depth of the feed material may vary from a maximum depth which is a function of the critically of the time versus temperature relation of the material to be treated to a minimum uniform depth of about one layer. It will also be appreciated that to increase the efficiency of the treatment of the feed material within the furnace, the feed material must also be uniformly distributed across the entire width of the rotating hearth as the feed material is placed upon the rotating hearth.
In addition to the problem of providing similar time versus temperature treatment of the feed material in a rotary hearth furnace, the rotary hearth surface inherently travels faster at the outer diameter than at the inner diameter because of the greater radius. Feed material is normally fed to the rotary hearth furnace at a constant flow rate. Accordingly, if the feed material is loaded at a uniform rate from the inner diameter to the outer diameter, the depth of the feed material will be greater at the inner diameter of the rotary hearth as compared to the outer diameter. Accordingly, it would be advantageous to provide a uniform feed material depth from the inner diameter to the outer diameter taking into consideration the difference in hearth surface speed.
In view of the foregoing, it is an object of the present invention to provide an improved charging apparatus for a rotary hearth furnace. Yet another object of the present invention is to provide a charging apparatus for a rotary hearth furnace that continuously distributes the feed material across the entire width of the hearth of the furnace. Still another object of the present invention is to provide a charging apparatus for a rotary hearth furnace that distributes the feed material to a uniform depth. It is another object of the present invention to provide a charging apparatus for a rotary hearth furnace that is simple and/or economical to manufacture and/or use.